Nonlinear evolution of electron shear flow instabilities in the presence of an external guide magnetic field (1608.02403v1)

Abstract: The dissipation mechanism by which the magnetic field reconnects in the presence of an external (guide) magnetic field in the direction of the main current is not well understood. In thin electron current sheets (ECS) (thickness ~ an electron inertial length) formed in collisionless magnetic reconnection, electron shear flow instabilities (ESFI) are potential candidates for providing an anomalous dissipation mechanism which can break the frozen-in condition of the magnetic field affecting the structure and rate of reconnection. We investigate the evolution of ESFI in guide field magnetic reconnection. The properties of the resulting plasma turbulence and their dependence on the strength of the guide field are studied. Utilizing 3-D electron-magnetohydrodynamic simulations of ECS we show that, unlike the case of ECS self-consistently embedded in anti-parallel magnetic fields, the evolution of thin ECS in the presence of a guide field (equal to the asymptotic value of the reconnecting magnetic field or larger) is dominated by high wave number non-tearing mode instabilities. The latter cause the development of, first, a wavy structure of the ECS. The turbulence, developed later, consists of current filaments and electron flow vortices. As a result of the nonlinear evolution of the instability, the ECS broadens simultaneously with its flattening in the central region mimicking a viscous-like turbulent dissipation. Later, the flattened ECS bifurcates. During the time of the bifurcation, the rate of the change of the mean electron flow velocity is proportional to the magnitude of the flow velocity, suggesting a resistive-like dissipation. The turbulence energy cascades to shorter wavelengths preferentially in the direction perpendicular to the guide magnetic field. The degree of anisotropy of the turbulence was found to increase with the increasing strength of the guide field.